In a recent article in
Nature, Mi and coworkers from the University of Michigan reported a solar-to-hydrogen (STH) efficiency of >9% in converting water into hydrogen and oxygen [
1], which represents an important breakthrough in this field due to the benchmarking leap in STH efficiency of photocatalytic overall water splitting under natural sunlight.
Photocatalytic overall water splitting toward hydrogen by utilizing renewable solar energy is regarded as a promising strategy to combat the critical challenges of climate change and the energy crisis arising from the excessive consumption of fossil resources [
2,
3]. Over the past decades, there has been a growing interest in developing efficient photocatalytic systems for splitting water into H
2 and O
2 with only the input of sunlight [
4]. The light-response range of the photocatalyst directly determines its theoretical maximum STH. Previously reported photocatalysts only respond to ultraviolet light and partially visible light. Eventually, the infrared light that accounts for more than 50% of solar energy is dissipated, which greatly limits their STH efficiency. In addition to inefficient light absorption and serious recombination of photogenerated electrons and holes, the sluggish reaction kinetics leads to limited success in overall water splitting toward hydrogen and oxygen at a stoichiometric ratio of 2:1, although the band structures of most of the semiconductors studied can straddle the redox potentials of water splitting. The undesirable backward reaction of hydrogen and oxygen into water remains one critical bottleneck for achieving a high STH as well. Therefore, in this field, despite remarkable achievements [
5–
7], the STH efficiency is far below the practical demand of 10%. Through the pioneering endeavor of Prof. Zetian Mi’s group at McGill University (Prof. Mi is currently a full professor at the University of Michigan Ann Arbor), III-nitrides nanowires-based photocatalysts have appeared as a promising light absorber for addressing the critical issues above [
8]. In recent years, considerable efforts have been devoted to improving the STH of III-nitrides nanowires-based photocatalysts by tuning the surface band structure [
9,
10], building internal electric field, and designing rational cocatalysts [
11], and a series of important progresses have been made. Most recently, as shown in Fig.1, utilizing Rh/Cr
2O
3/Co
3O
4-loaded InGaN/GaN nanowires fabricated by a well-developed combined method of plasma-assisted molecular beam epitaxy and
in situ photodeposition (Fig.1(a)), a significant progress in this field has been reported in
Nature by Prof. Zetian Mi’s group [
1]. This study demonstrates a world-record STH efficiency of approximately 9.2% with only the inputs of pure water, concentrated solar light, and InGaN/GaN nanowires-based photocatalyst. The outstanding performance is attributed to the following synergetic effects. First, the photocatalyst developed exhibits a wide visible-light response range (400–700 nm) for producing energetic photogenerated electrons and holes with suitable band-edge potentials for water splitting, which is the basic premise of a high STH. Meanwhile, the different InGaN segments in the photocatalyst does not form the commonly reported heterojunction or Z-scheme charge transfer but the unique multi-band structure (Fig.1(b)–1(c)). It is profitable for maximizing the redox ability of photogenerated electrons and holes with a high efficiency. More importantly, the photocatalyst of Rh/Cr
2O
3/Co
3O
4-loaded InGaN/GaN nanowires developed shows a remarkable dependence of STH on the conducting temperature of the system. The STH exhibits an increasing trend with the increasing temperature in a certain range of 30–70 °C, showing the viability of utilizing the thermal effect of long-wavelenth sunlight that cannot excite the semiconductor to improve the STH (Fig.1(d)). Guided by this discovery, infrared light is directly utilized to contribute to heating the reaction system (Fig.1(e)). It is found that the mass transfer and the chemical bond formation/cleavage can be enhanced in the reaction, thus benefiting a higher reaction rate. Meanwhile, according to the hydrogen-oxygen recombination experiment, the balance contents of hydrogen and oxygen increases with the increase of temperature and reaches the maximum value at approximately 70 °C (Fig.1(f)). Furthermore, the density functional theory (DFT) calculations indicate that the recombination reaction of hydrogen-oxygen on Rh is typically exothermal, and increasing temperature in a certain range can desirably inhibit the recombination (Fig.1(g)). However, excessive reaction temperature accelerates the diffusivity coefficient of hydrogen and oxygen, resulting in a high recombination. Therefore, there exists an optimal temperature of 70 °C for balancing backward reaction and mass transfer. As a result, the thermal effect of infrared light that heats the reaction system to 70 °C demonstrates a significant promotion in STH by accelerating the mass transfer and inhibiting the backward reaction of hydrogen and oxygen into water. Altogether, an unprecedented STH of 9.2% is achieved with an appreciable stability over Rh/Cr
2O
3/Co
3O
4-loaded InGaN/GaN nanowires photocatalyst in pure water under simulated sunlight light by a 300 W Xenon lamp equipped with an AM 1.5G filter (Fig.1(h)).
It is noted that to make the technology practically viable, the step equipped with a 4 cm × 4 cm wafer is tested outdoors by utilizing readily available water as hydrogen feedstocks (Fig.2(a)). The water can be readily converted into hydrogen with STH efficiencies of 6.2% without any applied bias and corrosive conductive electrolyzes (Fig.2(b)). Moreover, the system works perfectly under concentrated sunlight, which is highly beneficial for large-scale hydrogen at the low cost of photocatalyst using a limited land area.
Overall, this work demonstrates a significant breakthrough in STH of photocatalytic overall water splitting with a benchmarking value of 9.2% that approaches the threshold of commercialization. The outcomes give rise to a critical advance in this field, shedding light on the commercialization of hydrogen production from photocatalytic water splitting, especially from tap water and sea water under natural sunlight. To enable the practical application of this technique in the future, a higher STH of over 10% should be pursued by rational structure design of III-nitrides-based photocatalyst at atomic scale. The combination of III-nitrides-based photocatalyst with other semiconductors platforms with outstanding optoelectrical properties is worthy of consideration and efforts to further improve STH and stability. Specifically, a durability of up to thousands of days at a larger size such as 1 m × 1 m panel is required as well. Meanwhile, the price of device fabrication is also one key for commercial applications. For cocatalysts decoration, in situ photodeposition, electrodepostion, as well as other highly controllable methods, e.g., atomic layer deposition are very industry-friendly. Critically, the community looks forward to seeing further advances in semiconductor materials growth technologies such as molecular beam epitaxy, metal-organic chemical vapor deposition, and magnetic sputtering for significantly reducing the cost of device fabrication. In addition, to meet the practical requirements of safe storage, distribution and utilization, H2 should be purified from the H2/O2 mixture formed by developing high energy-efficiency and low-cost purification technologies. The combined advances in improving energy efficiency and stability and in lowering the device fabrication cost are highly critical to make green hydrogen production from water and sunlight more economically competitive in contrast to the convention route built on fossil fuels.
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